Disruption of influenza virus packaging signals results in various misassembled genome complexes

IMPORTANCE

The influenza A virus genome consists of eight distinct viral RNAs (vRNAs) that are typically packaged into a single virion as an octameric complex. How this genome complex is assembled and incorporated into the virion is poorly understood, but previous research suggests a coordinative role for packaging signals present in all vRNAs. Here, we show that disruption of two packaging signals in a model H7N7 influenza A virus results in a mixture of virions with unusual vRNA content, including empty virions, virions with one to four vRNAs, and virions with octameric complexes composed of vRNA duplicates. Our results suggest that (i) the assembly of error-free octameric complexes proceeds through a series of defined vRNA sub-complexes and (ii) virions can bud without incorporating complete octameric complexes.

Ebola and Marburg virus matrix layers are locally ordered assemblies of VP40 dimers

Filoviruses such as Ebola and Marburg virus bud from the host membrane as enveloped virions. This process is achieved by the matrix protein VP40. When expressed alone, VP40 induces budding of filamentous virus-like particles, suggesting that localization to the plasma membrane, oligomerization into a matrix layer, and generation of membrane curvature are intrinsic properties of VP40. There has been no direct information on the structure of VP40 matrix layers within viruses or virus-like particles. We present structures of Ebola and Marburg VP40 matrix layers in intact virus-like particles, and within intact Marburg viruses. VP40 dimers assemble extended chains via C-terminal domain interactions. These chains stack to form 2D matrix lattices below the membrane surface. These lattices form a patchwork assembly across the membrane and suggesting that assembly may begin at multiple points. Our observations define the structure and arrangement of the matrix protein layer that mediates formation of filovirus particles.

Adjuvant formulated virus-like particles expressing native-like forms of the Lassa virus envelope surface glycoprotein are immunogenic and induce antibodies with broadly neutralizing activity

Lassa mammarenavirus (LASV) is a rodent-borne arenavirus endemic to several West African countries. It is the causative agent of human Lassa fever, an acute viral hemorrhagic fever disease. To date, no therapeutics or vaccines against LASV have obtained regulatory approval. Polyclonal neutralizing antibodies derived from hyperimmunized animals may offer a useful strategy for prophylactic and therapeutic intervention to combat human LASV infections. The LASV envelope surface glycoprotein complex (GP) is the major target for neutralizing antibodies, and it is the main viral antigen used for the design of an LASV vaccine. Here, we assessed the immunogenic potential of mammalian cell-derived virus-like particles (VLPs) expressing GP from the prototypic LASV strain Josiah in a native-like conformation as the sole viral antigen. We demonstrate that an adjuvanted prime-boost immunization regimen with GP-derived VLPs elicited neutralizing antibody responses in rabbits, suggesting that effective antigenic epitopes of GP were displayed. Notably, these antibodies exhibited broad reactivity across five genetic lineages of LASV. VLP-based immunization strategies may represent a powerful approach for generating polyclonal sera containing cross-reactive neutralizing antibodies against LASV.

Nipah virus induces two inclusion body populations: Identification of novel inclusions at the plasma membrane

Formation of cytoplasmic inclusion bodies (IBs) is a hallmark of infections with non-segmented negative-strand RNA viruses (order Mononegavirales). We show here that Nipah virus (NiV), a bat-derived highly pathogenic member of the Paramyxoviridae family, differs from mononegaviruses of the Rhabdo-, Filo- and Pneumoviridae families by forming two types of IBs with distinct localizations, formation kinetics, and protein compositions. IBs in the perinuclear region form rapidly upon expression of the nucleocapsid proteins. These IB_peri are highly mobile and associate with the aggresome marker y-tubulin. IB_peri can recruit unrelated overexpressed cytosolic proteins but do not contain the viral matrix (M) protein. Additionally, NiV forms an as yet undescribed IB population at the plasma membrane (IB^PM) that is y-tubulin-negative but contains the M protein. Infection studies with recombinant NiV revealed that IB^PM require the M protein for their formation, and most likely represent sites of NiV assembly and budding. The identification of this novel type of plasma membrane-associated IBs not only provides new insights into NiV biology and may open new avenues to develop novel antiviral approaches to treat these highly pathogenic viruses, it also provides a basis for a more detailed characterization of IBs and their role in virus assembly and replication in infections with other Mononegavirales.

Inclusion bodies (IBs) induced by non-segmented negative-strand RNA viruses (Mononegavirales) are described as mobile cytosolic compartments that concentrate viral proteins and represent the main viral replication sites in infected cells. This general concept is mainly based on studies with mononegaviruses from the Rhabdo-, Filo- and Pneumoviridae families. IBs induced by members of the Paramyxoviridae family are much less well characterized, and this study provides evidence that paramyxoviral IBs may have different compositions and functions. The main finding of this study is that Nipah virus (NiV), a highly pathogenic member of the genus Henipavirus in the family Paramyxoviridae, forms a novel type of IB whose formation at plasma membrane assembly sites depends on the viral matrix protein, and suggests a role for IBs not yet described for other Mononegavirales. This discovery clearly extents the current concept of IB functions and illustrates the need to further investigate IBs formed by other paramyxoviruses.

Analysis of the multifunctionality of Marburg virus VP40

The Marburg virus (MARV) matrix protein, VP40, is a multifunctional protein that is essential for the assembly and release of viral particles, inhibition of the interferon response and viral transcription/replication. VP40 is assumed to be present as soluble monomers and membrane-bound higher-order oligomers. To investigate the functional relevance of oligomerization and lipid binding of VP40 we constructed mutants with impaired VP40-VP40 or VP40-lipid interactions and tested their capacity to bind the plasma membrane, to form virus-like particles (VLPs) and to inhibit viral RNA synthesis. All of the analysed VP40 mutants formed perinuclear aggregates and were defective in their delivery to the plasma membrane and in VLP production. The VP40 mutants that were competent for oligomerization but lacked VP40-lipid interactions formed fibril-like structures, influenced MARV inclusion body formation and inhibited viral transcription/replication more strongly than the VP40 wild-type. Altogether, mutations that interfere with VP40's transition from monomer to higher-order oligomers and/or lipid interactions destroy the protein's multifunctionality.

Ebola virus proteins NP, VP35, and VP24 are essential and sufficient to mediate nucleocapsid transport

The intracytoplasmic movement of nucleocapsids is a crucial step in the life cycle of enveloped viruses. Determination of the viral components necessary for viral nucleocapsid transport competency is complicated by the dynamic and complex nature of nucleocapsid assembly and the lack of appropriate model systems. Here, we established a live-cell imaging system based on the ectopic expression of fluorescent Ebola virus (EBOV) fusion proteins, allowing the visualization and analysis of the movement of EBOV nucleocapsid-like structures with different protein compositions. Only three of the five EBOV nucleocapsid proteins-nucleoprotein, VP35, and VP24-were necessary and sufficient to form transport-competent nucleocapsid-like structures. The transport of these structures was found to be dependent on actin polymerization and to have dynamics that were undistinguishable from those of nucleocapsids in EBOV-infected cells. The intracytoplasmic movement of nucleocapsid-like structures was completely independent of the viral matrix protein VP40 and the viral surface glycoprotein GP. However, VP40 greatly enhanced the efficiency of nucleocapsid recruitment into filopodia, the sites of EBOV budding.

Structure and assembly of the Ebola virus nucleocapsid

Ebola and Marburg viruses are filoviruses: filamentous, enveloped viruses that cause haemorrhagic fever. Filoviruses are within the order Mononegavirales, which also includes rabies virus, measles virus, and respiratory syncytial virus. Mononegaviruses have non-segmented, single-stranded negative-sense RNA genomes that are encapsidated by nucleoprotein and other viral proteins to form a helical nucleocapsid. The nucleocapsid acts as a scaffold for virus assembly and as a template for genome transcription and replication. Insights into nucleoprotein-nucleoprotein interactions have been derived from structural studies of oligomerized, RNA-encapsidating nucleoprotein, and cryo-electron microscopy of nucleocapsid or nucleocapsid-like structures. There have been no high-resolution reconstructions of complete mononegavirus nucleocapsids. Here we apply cryo-electron tomography and subtomogram averaging to determine the structure of Ebola virus nucleocapsid within intact viruses and recombinant nucleocapsid-like assemblies. These structures reveal the identity and arrangement of the nucleocapsid components, and suggest that the formation of an extended α-helix from the disordered carboxy-terminal region of nucleoprotein-core links nucleoprotein oligomerization, nucleocapsid condensation, RNA encapsidation, and accessory protein recruitment.

An active site mutation increases the polymerase activity of the guinea pig-lethal Marburg virus

Marburg virus (MARV) causes severe, often fatal, disease in humans and transient illness in rodents. Sequential passaging of MARV in guinea pigs resulted in selection of a lethal virus containing 4 aa changes. A D184N mutation in VP40 (VP40D184N), which leads to a species-specific gain of viral fitness, and three mutations in the active site of viral RNA-dependent RNA polymerase L, which were investigated in the present study for functional significance in human and guinea pig cells. The transcription/replication activity of L mutants was strongly enhanced by a substitution at position 741 (S741C), and inhibited by other substitutions (D758A and A759D) in both species. The polymerase activity of L carrying the S741C substitution was eightfold higher in guinea pig cells than in human cells upon co-expression with VP40D184N, suggesting that the additive effect of the two mutations provides MARV a replicative advantage in the new host.

Marburg virus inclusions: A virus-induced microcompartment and interface to multivesicular bodies and the late endosomal compartment

Filovirus infection of target cells leads to the formation of virally induced cytoplasmic inclusions that contain viral nucleocapsids at different stages of maturation. While the role of the inclusions has been unclear since the identification of Marburg and Ebola viruses, it recently became clear that the inclusions are the sites of viral replication, nucleocapsid formation and maturation. Live cell imaging analyses revealed that mature nucleocapsids are transported from inclusions to the filopodia, which represent the major budding sites. Moreover, inclusions recruit cellular proteins that have been shown to support the transport of nucleocapsids. For example, the tumor susceptibility gene 101 protein (Tsg101) interacts with a late domain motif in the nucleocapsid protein NP and recruits the actin-nucleation factor IQGAP1. Complexes of nucleocapsids together with Tsg101 and IQGAP1 are then co-transported along actin filaments. We detected additional proteins (Alix, Nedd4 and the AAA-type ATPase VPS4) of the endosomal sorting complex required for transport (ESCRT) that are recruited into inclusions. Together, the results suggest that nucleocapsids recruit the machinery that enhances viral budding at the plasma membrane. Furthermore, we identified Lamp1 as a marker of the late endosomal compartment in inclusions, while ER, Golgi, TGN and early endosomal markers were absent. In addition, we observed that LC3, a marker of autophagosomal membranes, was present in inclusions. The 3D structures of inclusions show an intricate structure that seems to accommodate an intimate cooperation between cellular and viral components with the intention to support viral transport and budding.

Transport of Ebolavirus Nucleocapsids Is Dependent on Actin Polymerization: Live-Cell Imaging Analysis of Ebolavirus-Infected Cells

BACKGROUND

Transport of ebolavirus (EBOV) nucleocapsids from perinuclear viral inclusions, where they are formed, to the site of budding at the plasma membrane represents an obligatory step of virus assembly. Until now, no live-cell studies on EBOV nucleocapsid transport have been performed, and participation of host cellular factors in this process, as well as the trajectories and speed of nucleocapsid transport, remain unknown.

METHODS

Live-cell imaging of EBOV-infected cells treated with different inhibitors of cellular cytoskeleton was used for the identification of cellular proteins involved in the nucleocapsid transport. EBOV nucleocapsids were visualized by expression of green fluorescent protein (GFP)-labeled nucleocapsid viral protein 30 (VP30) in EBOV-infected cells.

RESULTS

Incorporation of the fusion protein VP30-GFP into EBOV nucleocapsids was confirmed by Western blot and indirect immunofluorescence analyses. Importantly, VP30-GFP fluorescence was readily detectable in the densely packed nucleocapsids inside perinuclear viral inclusions and in the dispersed rod-like nucleocapsids located outside of viral inclusions. Live-cell imaging of EBOV-infected cells revealed exit of single nucleocapsids from the viral inclusions and their intricate transport within the cytoplasm before budding at the plasma membrane. Nucleocapsid transport was arrested upon depolymerization of actin filaments (F-actin) and inhibition of the actin-nucleating Arp2/3 complex, and it was not altered upon depolymerization of microtubules or inhibition of N-WASP. Actin comet tails were often detected at the rear end of nucleocapsids. Marginally located nucleocapsids entered filopodia, moved inside, and budded from the tip of these thin cellular protrusions.

CONCLUSIONS

Live-cell imaging of EBOV-infected cells revealed actin-dependent long-distance transport of EBOV nucleocapsids before budding at the cell surface. These findings provide useful insights into EBOV assembly and have potential application in the development of antivirals.

Interaction with Tsg101 is necessary for the efficient transport and release of nucleocapsids in marburg virus-infected cells

Endosomal sorting complex required for transport (ESCRT) machinery supports the efficient budding of Marburg virus (MARV) and many other enveloped viruses. Interaction between components of the ESCRT machinery and viral proteins is predominantly mediated by short tetrapeptide motifs, known as late domains. MARV contains late domain motifs in the matrix protein VP40 and in the genome-encapsidating nucleoprotein (NP). The PSAP late domain motif of NP recruits the ESCRT-I protein tumor susceptibility gene 101 (Tsg101). Here, we generated a recombinant MARV encoding NP with a mutated PSAP late domain (rMARV(PSAPmut)). rMARV(PSAPmut) was attenuated by up to one log compared with recombinant wild-type MARV (rMARV(wt)), formed smaller plaques and exhibited delayed virus release. Nucleocapsids in rMARV(PSAPmut)-infected cells were more densely packed inside viral inclusions and more abundant in the cytoplasm than in rMARV(wt)-infected cells. A similar phenotype was detected when MARV-infected cells were depleted of Tsg101. Live-cell imaging analyses revealed that Tsg101 accumulated in inclusions of rMARV(wt)-infected cells and was co-transported together with nucleocapsids. In contrast, rMARV(PSAPmut) nucleocapsids did not display co-localization with Tsg101, had significantly shorter transport trajectories, and migration close to the plasma membrane was severely impaired, resulting in reduced recruitment into filopodia, the major budding sites of MARV. We further show that the Tsg101 interacting protein IQGAP1, an actin cytoskeleton regulator, was recruited into inclusions and to individual nucleocapsids together with Tsg101. Moreover, IQGAP1 was detected in a contrail-like structure at the rear end of migrating nucleocapsids. Down regulation of IQGAP1 impaired release of MARV. These results indicate that the PSAP motif in NP, which enables binding to Tsg101, is important for the efficient actin-dependent transport of nucleocapsids to the sites of budding. Thus, the interaction between NP and Tsg101 supports several steps of MARV assembly before virus fission.

Actin filaments disruption and stabilization affect measles virus maturation by different mechanisms

Background

Cytoskeletal proteins are often involved in the virus life cycle, either at early steps during virus entry or at later steps during formation of new virus particles. Though actin filaments have been shown to play a role in the production of measles virus (MV), the importance of actin dynamics for virus assembly and budding steps is not known yet. Aim of this work was thus to analyze the distinctive consequences of F-actin stabilization or disruption for MV protein trafficking, particle assembly and virus release.

Results

MV infection studies in the presence of inhibitors differently affecting the actin cytoskeleton revealed that not only actin disruption but also stabilization of actin filaments interfered with MV particle release. While overall viral protein synthesis, surface expression levels of the MV glycoproteins, and cell-associated infectivity was not altered, cell-free virus titers were decreased. Interestingly, the underlying mechanisms of interference with late MV maturation steps differed principally after F-actin disruption by Cytochalasin D (CD) and F-actin stabilization by Jasplakinolide (Jaspla). While intact actin filaments were shown to be required for transport of nucleocapsids and matrix proteins (M-RNPs) from inclusions to the plasma membrane, actin dynamics at the cytocortex that are blocked by Jaspla are necessary for final steps in virus assembly, in particular for the formation of viral buds and the pinching-off at the plasma membrane. Supporting our finding that F-actin disruption blocks M-RNP transport to the plasma membrane, cell-to-cell spread of MV infection was enhanced upon CD treatment. Due to the lack of M-glycoprotein-interactions at the cell surface, M-mediated fusion downregulation was hindered and a more rapid syncytia formation was observed.

Conclusion

While stable actin filaments are needed for intracellular trafficking of viral RNPs to the plasma membrane, and consequently for assembly at the cell surface and prevention of an overexerted fusion by the viral surface glycoproteins, actin dynamics are required for the final steps of budding at the plasma membrane.

Influenza virus budding from the tips of cellular microvilli in differentiated human airway epithelial cells

The epithelium of conducting airways represents the main target for influenza virus in mammals. However, the peculiarities of virus interactions with differentiated airway epithelial cells remain largely unknown. Here, influenza virus budding was studied in differentiated cultures of human tracheobronchial epithelial cells using transmission electron microscopy. Budding of spherical and filamentous virions was observed on the apical surfaces of cells with no association with cilia and secretory granules. Quantitative analysis of the distribution of viral buds on the cell surface indicated that the tips of the microvilli represented a prominent site of influenza virus budding in the human airway epithelium. As the microvilli of differentiated cells are involved in many fundamental cell functions, these data will prompt further studies on the biological significance of microvilli-associated budding for virus replication, transmission and pathogenicity.

Assembly of the Marburg virus envelope

The key player to assemble the filamentous Marburg virus particles is the matrix protein VP40 which orchestrates recruitment of nucleocapsid complexes and the viral glycoprotein GP to the budding sites at the plasma membrane. Here, VP40 induces the formation of the viral particles, determines their morphology and excludes cellular proteins from the virions. Budding takes place at filopodia in non-polarized cells and at the basolateral cell pole in polarized epithelial cells. Molecular basis of how VP40 exerts its multifunctional role in these different processes is currently under investigation. Here we summarize recent data on structure-function relationships of VP40 and GP in connection with their function in assembly. Questions concerning the complex particle assembly, budding and release remaining enigmatic are addressed. Cytoplasmic domains of viral surface proteins often serve as a connection to the viral matrix protein or as binding sites for further viral or cellular proteins. A cooperation of MARV GP and VP40 building up the viral envelope can be proposed and is discussed in more detail in this review, as the cytoplasmic domain of GP represents an obvious interaction candidate because of its localization adjacent to the VP40 layer. Interestingly, truncation of the short cytoplasmic domain of GP neither inhibited interaction with VP40 nor incorporation of GP into progeny viral particles. Based on reverse genetics we generated recombinant virions expressing a GP mutant without the cytoplasmic tail. Investigations revealed attenuation in virus growth and an obvious defect in entry. Further investigations showed that the truncation of the cytoplasmic domain of GP impaired the structural integrity of the ectodomain, whichconsequently had impact on entry steps downstream of virus binding. Our data indicated that changes in the cytoplasmic domain are relayed over the lipid membrane to alter the function of the ectodomain.

Exploring synergies between academia and vaccine manufacturers: a pilot study on how to rapidly produce vaccines to combat emerging pathogens

BACKGROUND

In spring 2009, a new swine-origin influenza A (H1N1) virus emerged in Mexico. During the following weeks the virus spread worldwide, prompting the World Health Organization to declare the first influenza pandemic of the 21st century. Sustained human-to-human transmission and severe disease progression observed in some patients urged public health authorities to respond rapidly to the disease outbreak and vaccine manufacturers to develop pandemic influenza vaccines for mass distribution. With the onset of the pandemic we began to explore the potential of academic/industrial collaboration to accelerate the production of vaccines during an outbreak of an emerging virus by combining the use of an academic BSL-4 laboratory with the expertise of a commercial vaccine manufacturer.

METHODS AND RESULTS

To obtain virus seed stocks used for the production of a vaccine to combat the pandemic H1N1 2009 influenza virus (H1N1pdm), we followed various strategies: (i) optimization of cell culture conditions for growth of wild-type H1N1pdm isolates; (ii) classical reassortment of H1N1pdm and standard influenza vaccine donor strain PR8; and (iii) generation of corresponding reassortant viruses using reverse genetics. To ensure a rapid transition to production, the entire potential seed stock development process was carried out in a certified canine kidney suspension cell line (MDCK 33016-PF) under Good Manufacturing Practice (GMP) conditions.

CONCLUSIONS

The outcome of this study indicates that a combination of different experimental strategies is the best way to cope with the need to develop vaccines rapidly in the midst of an emerging pandemic.

The importance of the NP: VP35 ratio in Ebola virus nucleocapsid formation

Ebola virus VP35 is a cofactor of the viral RNA polymerase complex and, together with NP and VP24, is an essential component for nucleocapsid formation. In the present study, we examined the interactions between VP35 and NP and found that VP35 interacts with helical NP-RNA complexes through the C-terminus of NP. We also found that coexpression of excess VP35 with NP reduced the yields of NP-RNA complexes purified by CsCl gradient ultracentrifugation and inhibited the formation of the NP-induced inclusion bodies that typically form in Ebola virus-infected cells. These findings suggest that the NP to VP35 ratio is important in the Ebola virus replication cycle and advance our knowledge of nucleocapsid morphogenesis.

Cryo-electron tomography of Marburg virus particles and their morphogenesis within infected cells

Several major human pathogens, including the filoviruses, paramyxoviruses, and rhabdoviruses, package their single-stranded RNA genomes within helical nucleocapsids, which bud through the plasma membrane of the infected cell to release enveloped virions. The virions are often heterogeneous in shape, which makes it difficult to study their structure and assembly mechanisms. We have applied cryo-electron tomography and sub-tomogram averaging methods to derive structures of Marburg virus, a highly pathogenic filovirus, both after release and during assembly within infected cells. The data demonstrate the potential of cryo-electron tomography methods to derive detailed structural information for intermediate steps in biological pathways within intact cells. We describe the location and arrangement of the viral proteins within the virion. We show that the N-terminal domain of the nucleoprotein contains the minimal assembly determinants for a helical nucleocapsid with variable number of proteins per turn. Lobes protruding from alternate interfaces between each nucleoprotein are formed by the C-terminal domain of the nucleoprotein, together with viral proteins VP24 and VP35. Each nucleoprotein packages six RNA bases. The nucleocapsid interacts in an unusual, flexible "Velcro-like" manner with the viral matrix protein VP40. Determination of the structures of assembly intermediates showed that the nucleocapsid has a defined orientation during transport and budding. Together the data show striking architectural homology between the nucleocapsid helix of rhabdoviruses and filoviruses, but unexpected, fundamental differences in the mechanisms by which the nucleocapsids are then assembled together with matrix proteins and initiate membrane envelopment to release infectious virions, suggesting that the viruses have evolved different solutions to these conserved assembly steps.

Genus-specific recruitment of filovirus ribonucleoprotein complexes into budding particles

The filoviral matrix protein VP40 orchestrates virus morphogenesis and budding. To do this it interacts with both the glycoprotein (GP1,2) and the ribonucleoprotein (RNP) complex components; however, these interactions are still not well understood. Here we show that for efficient VP40-driven formation of transcription and replication-competent virus-like particles (trVLPs), which contain both an RNP complex and GP1,2, the RNP components and VP40, but not GP1,2 and VP40, must be from the same genus. trVLP preparations contained both spherical and filamentous particles, but only the latter were able to infect target cells and to lead to genome replication and transcription. Interestingly, the genus specificity of the VP40-RNP interactions was specific to the formation of filamentous trVLPs, but not to spherical particles. These results not only further our understanding of VP40 interactions, but also suggest that special care is required when using trVLP or VLP systems to model virus morphogenesis.

Fractional nonablative 1540-nm laser treatment of striae distensae in Fitzpatrick skin types II to IV: clinical and histological results

BACKGROUND

Current striae treatments are limited in their ability to deliver long-lasting improvements for all skin types. The success of fractional nonablative lasers for surgical scars has been attributed to the controlled wound-healing response stimulated by microscopic columns of epidermal and dermal thermal damage.

OBJECTIVES

The authors describe the safety and efficacy results of treatment with a fractional nonablative 1540-nm erbium:glass laser in patients with Fitzpatrick skin types II to IV for both striae rubra and striae alba.

METHODS

A 51-person clinical study was conducted on striae ranging in duration from one to 40 years. Nine different anatomical locations were treated, including the breasts, hips, and abdomen. Treatment parameters included two to three passes with the 1540-nm laser, with energy settings from 35 to 55 mJ/mb with the 10-mm optical tip or 12 to 14 mJ/mb with the 15-mm optical tip. Two to four total treatments were performed at four- to six-week intervals. Nonblinded efficacy evaluations were performed on all 51 patients; blinded evaluations were conducted by three independent clinicians on 14 randomized sets of pre- and posttreatment images on a 0% to 100% quartile improvement scale. Skin reactions were assessed by the treating physician and recorded at multiple time points, and histology was conducted with hemotoxylin and eosin as well as Orcein-Giemsa staining.

RESULTS

Nonblinded clinical assessments rated overall improvement as 50% or greater for all patients at six months or longer after the last treatment. Blinded evaluators reported an overall mean improvement score of 51% to 75% on properly selected images taken at least three months after treatment (n = 11). In all patients examined at either 18 or 24 months after treatment, there was no recurrence of striae. Typical side effects included transient erythema and edema. A small minority of patients experienced transient cases of trace postinflammatory hyperpigmentation (PIH), which all resolved. Histologic observations showed thickening of the epidermis and dermis, neocollagenesis, and increased elastin deposition one month after the last treatment.

CONCLUSIONS

Positive safety and efficacy results with the fractional nonablative 1540-nm erbium:glass laser for the treatment of striae rubra and striae alba ranging in maturation age from one to 40 years was demonstrated in Fitzpatrick skin types II to IV.

Establishment of fruit bat cells (Rousettus aegyptiacus) as a model system for the investigation of filoviral infection

Background

The fruit bat species Rousettus aegyptiacus was identified as a potential reservoir for the highly pathogenic filovirus Marburg virus. To establish a basis for a molecular understanding of the biology of filoviruses in the reservoir host, we have adapted a set of molecular tools for investigation of filovirus replication in a recently developed cell line, R06E, derived from the species Rousettus aegyptiacus.

Methodology/Principal Findings

Upon infection with Ebola or Marburg viruses, R06E cells produced viral titers comparable to VeroE6 cells, as shown by TCID₅₀ analysis. Electron microscopic analysis of infected cells revealed morphological signs of filovirus infection as described for human- and monkey-derived cell lines. Using R06E cells, we detected an unusually high amount of intracellular viral proteins, which correlated with the accumulation of high numbers of filoviral nucleocapsids in the cytoplasm. We established protocols to produce Marburg infectious virus-like particles from R06E cells, which were then used to infect naïve target cells to investigate primary transcription. This was not possible with other cell lines previously tested. Moreover, we established protocols to reliably rescue recombinant Marburg viruses from R06E cells.

Conclusion/Significance

These data indicated that R06E cells are highly suitable to investigate the biology of filoviruses in cells derived from their presumed reservoir.

Marburg virus and several species of Ebola virus are endemic in central Africa and cause sporadic outbreaks in this region with mortality rates of up to 90%. So far, there is no vaccination or therapy available to protect people at risk in these regions. Recently, different fruit bats have been identified as potential reservoirs. One of them is Rousettus aegyptiacus. It seems that within huge bat populations only relatively small numbers are positive for filovirus-specific antibodies or filoviral RNA, a phenomenon that is currently not understood. As a first step towards understanding the biology of filoviruses in bats, we sought to establish a model system to investigate filovirus replication in cells derived from their natural reservoir. Here, we provide the first insights into this topic by monitoring filovirus infection of a Rousettus aegyptiacus derived cell line, R06E. We were able to show that filoviruses propagate well in R06E cells, which can, therefore, be used to investigate replication and transcription of filovirus RNA and to very efficiently perform rescue of recombinant Marburg virus using reverse genetics. These results emphasize the suitability of the newly established bat cell line for filovirus research.

Fatal Cowpox Virus Infection in Captive Banded Mongooses (Mungos mungo)

Cowpox virus infections have been described in various domestic and exotic animal species. This report is the first on an outbreak of fatal generalized cowpox virus infection among captive banded mongooses ( Mungos mungo, suborder Feliformia). All animals of a colony of 8 mongooses showed a fulminant course of disease. The whole population died (n=7) or was euthanized (n=1) within 11 days. Postmortem examinations were performed on 4 animals. All animals showed extensive necrotizing inflammation of retropharyngeal lymph nodes, typical poxviral skin lesions, and multiple necrotic foci in liver and spleen. Three animals exhibited an ulcerating stomatitis. Pulmonary lesions, a common feature of fatal cowpox virus infections in other feliform species, were not obvious. Histopathologically, characteristic cytoplasmic inclusion bodies were detected in all affected organs but the spleen. Based on transmission electron microscopy and cell culture, Orthopoxvirus was identified as the etiology. The virus was further characterized by polymerase chain reaction and sequence analysis, identifying it as cowpox virus. A survey in the habitat suggests wild brown rats ( Rattus norvegicus) as the most likely source of infection.

Electron tomography reveals the steps in filovirus budding

The filoviruses, Marburg and Ebola, are non-segmented negative-strand RNA viruses causing severe hemorrhagic fever with high mortality rates in humans and nonhuman primates. The sequence of events that leads to release of filovirus particles from cells is poorly understood. Two contrasting mechanisms have been proposed, one proceeding via a “submarine-like” budding with the helical nucleocapsid emerging parallel to the plasma membrane, and the other via perpendicular “rocket-like” protrusion. Here we have infected cells with Marburg virus under BSL-4 containment conditions, and reconstructed the sequence of steps in the budding process in three dimensions using electron tomography of plastic-embedded cells. We find that highly infectious filamentous particles are released at early stages in infection. Budding proceeds via lateral association of intracellular nucleocapsid along its whole length with the plasma membrane, followed by rapid envelopment initiated at one end of the nucleocapsid, leading to a protruding intermediate. Scission results in local membrane instability at the rear of the virus. After prolonged infection, increased vesiculation of the plasma membrane correlates with changes in shape and infectivity of released viruses. Our observations demonstrate a cellular determinant of virus shape. They reconcile the contrasting models of filovirus budding and allow us to describe the sequence of events taking place during budding and release of Marburg virus. We propose that this represents a general sequence of events also followed by other filamentous and rod-shaped viruses.

The filoviruses, Marburg and Ebola, cause lethal hemorrhagic fever and are highest-priority bioterrorism agents. Filovirus particles contain a rod-like nucleocapsid and are normally filamentous, though other shapes are seen. It is poorly understood how such large filamentous particles are assembled and released from infected cells. Here we have studied Marburg virus production in infected cells using electron tomography. This technique allows virus particles to be visualized in three dimensions at different stages during assembly. We find that in early stages of virus production, highly infectious filamentous viruses are produced, whereas after prolonged infection poorly infectious spherical viruses are released. We also define the sequence of steps in filamentous virus release. The intracellular nucleocapsid first travels to the plasma membrane of the cell, where it binds laterally along its whole length. One end is then wrapped by the plasma membrane and wrapping proceeds rapidly until the virus protrudes vertically from the cell surface. The rear end of the virus particle then pinches off from the cell. We propose that other important filamentous and rod-shaped viruses also follow this series of steps of assembly and budding.

Measles virus M protein-driven particle production does not involve the endosomal sorting complex required for transport (ESCRT) system

Assembly and budding of enveloped RNA viruses rely on viral matrix (M) proteins and host proteins involved in sorting and vesiculation of cellular cargoes, such as the endosomal sorting complex required for transport (ESCRT). The measles virus (MV) M protein promotes virus-like particle (VLP) production, and we now show that it shares association with detergent-resistant or tetraspanin-enriched membrane microdomains with ebolavirus VP40 protein, yet accumulates less efficiently at the plasma membrane. Unlike VP40, which recruits ESCRT components via its N-terminal late (L) domain and exploits them for particle production, the M protein does this independently of this pathway, as (i) ablation of motifs bearing similarity to canonical L domains did not affect VLP production, (ii) it did not redistribute Tsg101, AIP-1 or Vps4A to the plasma membrane, and (iii) neither VLP nor infectious virus production was sensitive to inhibition by dominant-negative Vps4A. Importantly, transfer of the VP40 L domain into the MV M protein did not cause recruitment of ESCRT proteins or confer sensitivity of VLP release to Vps4A, indicating that MV particle production occurs independently of and cannot be routed into an ESCRT-dependent pathway.

Establishment and application of an infectious virus-like particle system for Marburg virus

The highly pathogenic Marburg virus (MARV) can only be investigated in high containment laboratories, which is time consuming and expensive. To investigate the MARV life cycle under normal laboratory conditions, an infectious virus-like particle (VLP) system was developed. The infectious VLP system is based on the T7-polymerase driven synthesis of a MARV-specific minigenome that encodes luciferase and is transcribed and replicated by the simultaneously expressed MARV nucleocapsid proteins NP, VP35, L and VP30. Transcription of the minigenome resulted in luciferase activity and replication resulted in encapsidated minigenomes. The encapsidated minigenomes, together with the viral matrix proteins VP40 and VP24 and the surface glycoprotein (GP), formed VLPs at the plasma membrane. Among the released pleomorphic VLPs, filamentous particles of 200-400 nm in length showed the highest capacity to induce reporter activity upon infection of target cells. To characterize the infectious VLP system, the intracellular concentration of one of the components was titrated, while all others were held constant. Intracellular concentrations of nucleocapsid proteins that resulted in highest replication and transcription activities also yielded VLPs with the highest ability to induce luciferase activity in target cells. High intracellular levels of VP40 maximized the release of VLPs, but reduced their ability to induce luciferase activity in target cells. The intracellular concentration of GP positively correlated with its incorporation into VLPs and their infectivity. Finally, we demonstrated that the infectious VLP system was suitable for rapid screening of neutralizing antibodies directed against MARV.

Filoviruses: Interactions with the host cell

The highly pathogenic filoviruses, Marburg and Ebola virus, are difficult to handle and knowledge of the interactions between filoviruses and their host cells remained enigmatic for many years. Two developments were crucial for the presented advances in our understanding of the cell biology of filoviruses, which is still fragmentary. On the one hand, the number of high containment laboratories increased where handling of the highly pathogenic filoviruses is possible. On the other hand, molecular biological tools have been developed that allow investigation of certain aspects of filoviral replication under normal laboratory conditions which considerably accelerated research on filoviruses. This review describes advances in understanding the interactions between host cells and filoviruses during viral attachment, entry, transcription, assembly and budding.

Nucleocapsid formation and RNA synthesis of Marburg virus is dependent on two coiled coil motifs in the nucleoprotein

The nucleoprotein (NP) of Marburg virus (MARV) is responsible for the encapsidation of viral genomic RNA and the formation of the helical nucleocapsid precursors that accumulate in intracellular inclusions in infected cells. To form the large helical MARV nucleocapsid, NP needs to interact with itself and the viral proteins VP30, VP35 and L, which are also part of the MARV nucleocapsid. In the present study, a conserved coiled coil motif in the central part of MARV NP was shown to be an important element for the interactions of NP with itself and VP35, the viral polymerase cofactor. Additionally, the coiled coil motif was essential for the formation of NP-induced intracellular inclusions and for the function of NP in the process of transcription and replication of viral RNA in a minigenome system. Transfer of the coiled coil motif to a reporter protein was sufficient to mediate interaction of the constructed fusion protein with the N-terminus of NP. The coiled coil motif is bipartite, constituted by two coiled coils which are separated by a flexible linker.

Recent advances in filovirus- and arenavirus-like particles

Both filoviruses and arenaviruses are enveloped viruses with a single-stranded RNA genome. Most human pathogenic arenaviruses, as well as filoviruses, cause severe hemorrhagic fevers with a high rate of case fatalities. Increasing numbers of outbreaks, the possibility of imported infections and the potential use of these viruses as bioterrorism agents have led to increased interest in these viruses and their biology. Virus-like particles are excellent tools to study the life-cycle of filoviruses and arenaviruses and have demonstrated the potential for use as safe and effective vaccines. This review summarizes the recent advances in the production, study and application of filovirus- and arenavirus-like particles, and provides an outlook on possible future directions for research into these viruses using virus-like particles.

Role of the transmembrane domain of marburg virus surface protein GP in assembly of the viral envelope

The major protein constituents of the filoviral envelope are the matrix protein VP40 and the surface transmembrane protein GP. While VP40 is recruited to the sites of budding via the late retrograde endosomal transport route, GP is suggested to be transported via the classical secretory pathway involving the endoplasmic reticulum, Golgi apparatus, and trans-Golgi network until it reaches the plasma membrane where most filoviral budding takes place. Since both transport routes target the plasma membrane, it was thought that GP and VP40 join there to form the viral envelope. However, it was recently shown that, upon coexpression of both proteins, GP is partially recruited into peripheral VP40-enriched multivesicular bodies, which contained markers of the late endosome. Accumulation of GP and VP40 in this compartment was presumed to play an important role in the formation of the filoviral envelope. Using a domain-swapping approach, we were able to show that the transmembrane domain of GP was essential and sufficient for (i) partial recruitment of chimeric glycoproteins into VP40-enriched multivesicular bodies and (ii) incorporation into virus-like particles (VLPs) that were released upon expression of VP40. Only those chimeric glycoproteins which were targeted to VP40-enriched endosomal multivesicular bodies were subsequently recruited into VLPs. These data show that the transmembrane domain of GP is critical for the mixing of VP40 and GP in multivesicular bodies and incorporation of GP into the viral envelope. Results further suggest that trapping of GP in the VP40-enriched late endosomal compartment is important for the formation of the viral envelope.

Budding of Marburgvirus is associated with filopodia

Viruses exploit the cytoskeleton of host cells to transport their components and spread to neighbouring cells. Here we show that the actin cytoskeleton is involved in the release of Marburgvirus (MARV) particles. We found that peripherally located nucleocapsids and envelope precursors of MARV are located either at the tip or at the side of filopodial actin bundles. Importantly, viral budding was almost exclusively detected at filopodia. Inhibiting actin polymerization in MARV-infected cells significantly diminished the amount of viral particles released into the medium. This suggested that dynamic polymerization of actin in filopodia is essential for efficient release of MARV. The viral matrix protein VP40 plays a key role in the release of MARV particles and we found that the intracellular localization of recombinant VP40 and its release in form of virus-like particles were strongly influenced by overexpression or inhibition of myosin 10 and Cdc42, proteins important in filopodia formation and function. We suggest that VP40, which is capable of interacting with viral nucleocapsids, provides an interface of MARV subviral particles and filopodia. As filopodia are in close contact with neighbouring cells, usurpation of these structures may facilitate spread of MARV to adjacent cells.

Infection of naive target cells with virus-like particles: implications for the function of ebola virus VP24

Infectious virus-like particle (iVLP) systems have recently been established for several negative-strand RNA viruses, including the highly pathogenic Zaire ebolavirus (ZEBOV), and allow study of the viral life cycle under biosafety level 2 conditions. However, current systems depend on the expression of viral helper nucleocapsid proteins in target cells, thus making it impossible to determine whether ribonucleoprotein complexes transferred by iVLPs are able to facilitate initial transcription, an indispensable step in natural infection. Here we describe a ZEBOV iVLP system which overcomes this limitation and show that VP24 is essential for the formation of a functional ribonucleoprotein complex.

Rescue of recombinant Marburg virus from cDNA is dependent on nucleocapsid protein VP30

Here we report recovery of infectious Marburg virus (MARV) from a full-length cDNA clone. Compared to the wild-type virus, recombinant MARV showed no difference in terms of morphology of virus particles, intracellular distribution in infected cells, and growth kinetics. The nucleocapsid protein VP30 of MARV and Ebola virus (EBOV) contains a Zn-binding motif which is important for the function of VP30 as a transcriptional activator in EBOV, whereas its role for MARV is unclear. It has been reported previously that MARV VP30 is able to support transcription in an EBOV-specific minigenome system. When the Zn-binding motif was destroyed, MARV VP30 was shown to be inactive in the EBOV system. While it was not possible to rescue recombinant MARV when the VP30 plasmid was omitted from transfection, MARV VP30 with a destroyed Zn-binding motif and EBOV VP30 were able to mediate virus recovery. In contrast, rescue of recombinant EBOV was not supported by EBOV VP30 containing a mutated Zn-binding domain.

VP24 of Marburg virus influences formation of infectious particles

The highly pathogenic enveloped Marburg virus (MARV) is composed of seven structural proteins and the nonsegmented negative-sense viral RNA genome. Four proteins (NP, VP35, VP30, and L) make up the helical nucleocapsid, which is surrounded by a matrix that is composed of the viral proteins VP40 and VP24. VP40 is functionally homologous to the matrix proteins of other nonsegmented negative-strand RNA viruses. As yet, the function of VP24 remains elusive. In the present study we found that VP24 colocalized with inclusions in MARV-infected cells that contain preformed nucleocapsids and with nucleocapsids outside the inclusions. Coexpression studies revealed that VP24 is recruited into the inclusions by the presence of NP. Furthermore, VP24 displayed membrane-binding properties and was recruited into filamentous virus-like particles (VLPs) that are induced by VP40. The incorporation of VP24 altered neither the morphology of VLPs nor the budding efficiency of VLPs. When VP24 was silenced in MARV-infected cells by small interfering RNA technology, the release of viral particles was significantly reduced while viral transcription and replication were unimpaired. Our data support the idea that VP24 is essential for a process that takes place after replication and transcription and before budding of virus progeny. It is presumed that VP24 is necessary for the formation of transport-competent nucleocapsids and/or the interaction between the nucleocapsids and the budding sites at the plasma membrane.

A chimeric respiratory syncytial virus fusion protein functionally replaces the F and HN glycoproteins in recombinant Sendai virus

Entry of most paramyxoviruses is accomplished by separate attachment and fusion proteins that function in a cooperative manner. Because of this close interdependence, it was not possible with most paramyxoviruses to replace either of the two protagonists by envelope glycoproteins from related paramyxoviruses. By using reverse genetics of Sendai virus (SeV), we demonstrate that chimeric respiratory syncytial virus (RSV) fusion proteins containing either the cytoplasmic domain of the SeV fusion protein or in addition the transmembrane domain were efficiently incorporated into SeV particles provided the homotypic SeV-F was deleted. In the presence of SeV-F, the chimeric glycoproteins were incorporated with significantly lower efficiency, indicating that determinants in the SeV-F ectodomain exist that contribute to glycoprotein uptake. Recombinant SeV in which the homotypic fusion protein was replaced with chimeric RSV fusion protein replicated in a trypsin-independent manner and was neutralized by antibodies directed to RSV-F. However, replication of this virus also relied on the hemagglutinin-neuraminidase (HN) as pretreatment of cells with neuraminidase significantly reduced the infection rate. Finally, recombinant SeV was generated with chimeric RSV-F as the only envelope glycoprotein. This virus was not neutralized by antibodies to SeV and did not use sialic acids for attachment. It replicated more slowly than hybrid virus containing HN and produced lower virus titers. Thus, on the one hand RSV-F can mediate infection in an autonomous way while on the other hand it accepts support by a heterologous attachment protein.

Radiotherapy of skin carcinomas of the pinna: a study of 115 lesions in 108 patients

BACKGROUND

The possibility of treating skin carcinomas of the pinna with radiotherapy is somewhat under discussion and scarcely known. Therefore the aim of the study was to evaluate the effectiveness and safety of dermatologic radiotherapy in a series of patients affected by basal or squamous cell carcinoma of the pinna.

METHODS

A retrospective study was performed on 108 patients affected by 115 carcinomas of the pinna (99 basal cell carcinomas, 16 squamous cell carcinomas) without involvement of the external auditory canal. Radiotherapy was performed with kilovoltage techniques (55-120 kV) and the total doses administered ranged from 45 to 70 Gy (105 Gy in one case only), with different fractionations.

RESULTS

The mean follow-up was 28.80 months. Complete remission was obtained in 111 lesions (96.52%) and partial remission in one (0.87%), as evaluated 1 month after the end of radiotherapy. No response was observed in two lesions (1.74%). The response was not evaluable in one lesion (0.87%). During follow up a relapse was observed in 12 lesions (all basal cell carcinomas): nine central and three marginal to the irradiation field. The 5-year cure-rate from the end of radiotherapy was 78%. The cosmetic results were evaluated as good or acceptable in 88.28% of lesions. No complications nor sequelae to the treatment were observed.

CONCLUSIONS

The results obtained confirm the possibility of treating epithelial skin neoplasms of the pinna with dermatologic radiotherapy, which can afford high-remission percentages without damaging cartilaginous tissue.

VP40 octamers are essential for Ebola virus replication

Matrix protein VP40 of Ebola virus is essential for virus assembly and budding. Monomeric VP40 can oligomerize in vitro into RNA binding octamers, and the crystal structure of octameric VP40 has revealed that residues Phe125 and Arg134 are the most important residues for the coordination of a short single-stranded RNA. Here we show that full-length wild-type VP40 octamers bind RNA upon HEK 293 cell expression. While the Phe125-to-Ala mutation resulted in reduced RNA binding, the Arg134-to-Ala mutation completely abolished RNA binding and thus octamer formation. The absence of octamer formation, however, does not affect virus-like particle (VLP) formation, as the VLPs generated from the expression of wild-type VP40 and mutated VP40 in HEK 293 cells showed similar morphology and abundance and no significant difference in size. These results strongly indicate that octameric VP40 is dispensable for VLP formation. The cellular localization of mutant VP40 was different from that of wild-type VP40. While wild-type VP40 was present in small patches predominantly at the plasma membrane, the octamer-negative mutants were found in larger aggregates at the periphery of the cell and in the perinuclear region. We next introduced the Arg134-to-Ala and/or the Phe125-to-Ala mutation into the Ebola virus genome. Recombinant wild-type virus and virus expressing the VP40 Phe125-to-Ala mutation were both rescued. In contrast, no recombinant virus expressing the VP40 Arg134-to-Ala mutation could be recovered. These results suggest that RNA binding of VP40 and therefore octamer formation are essential for the Ebola virus life cycle.

Multivesicular bodies as a platform for formation of the Marburg virus envelope

The Marburg virus (MARV) envelope consists of a lipid membrane and two major proteins, the matrix protein VP40 and the glycoprotein GP. Both proteins use different intracellular transport pathways: GP utilizes the exocytotic pathway, while VP40 is transported through the retrograde late endosomal pathway. It is currently unknown where the proteins combine to form the viral envelope. In the present study, we identified the intracellular site where the two major envelope proteins of MARV come together as peripheral multivesicular bodies (MVBs). Upon coexpression with VP40, GP is redistributed from the trans-Golgi network into the VP40-containing MVBs. Ultrastructural analysis of MVBs suggested that they provide the platform for the formation of membrane structures that bud as virus-like particles from the cell surface. The virus-like particles contain both VP40 and GP. Single expression of GP also resulted in the release of particles, which are round or pleomorphic. Single expression of VP40 led to the release of filamentous structures that closely resemble viral particles and contain traces of endosomal marker proteins. This finding indicated a central role of VP40 in the formation of the filamentous structure of MARV particles, which is similar to the role of the related Ebola virusVP40. In MARV-infected cells, VP40 and GP are colocalized in peripheral MVBs as well. Moreover, intracellular budding of progeny virions into MVBs was frequently detected. Taken together, these results demonstrate an intracellular intersection between GP and VP40 pathways and suggest a crucial role of the late endosomal compartment for the formation of the viral envelope.

Site directed mutagenesis of the carboxyl terminus of human cytomegalovirus glycoprotein B leads to attenuation of viral growth in cell culture

A viable human cytomegalovirus (HCMV) mutant was generated harbouring a glycoprotein B (gB) in which the carboxyl-terminal amino acids DRLRHR (aa 885-900) were changed to AALREE. Characterization of the phenotype of the recombinant virus revealed significant reduction of infectious progeny release and only moderate reduction of viral DNA replication indicating its diminished specific infectivity. This observation was in line with immunogold labeling of extracellular virions demonstrating that the amount of gB protein was markedly reduced in the envelope of the mutant virus. Our results suggest that the conserved carboxyl-terminus of the gB molecule is critical for HCMV maturation.

Characterization of the Lassa virus matrix protein Z: electron microscopic study of virus-like particles and interaction with the nucleoprotein (NP)

Lassa virus is the causative agent of a hemorrhagic fever endemic in west Africa. The RNA genome of Lassa virus encodes the glycoprotein precursor GP-C, a nucleoprotein (NP), the viral polymerase L and a small protein Z (11 kDa). Here, we analyze the role of Z protein for virus maturation. We have recently shown that expression of Z protein in the absence of other viral proteins is sufficient for the release of enveloped Z-containing particles. In this study, we examined particles secreted into the supernatant of a stably Z protein-expressing CHO cell line by electron microscopy. The observed Z-induced virus-like particles did not significantly differ in their morphology and size from Lassa virus particles. Mutation of two proline-rich domains within Z which are known to drastically reduce the release of virus-like particles, had no effect on the cellular localization of the protein nor on its membrane-association. Furthermore, we present evidence that Z interacts with the NP. We assume that Z recruits NP to cellular membranes where virus assembly takes place. We conclude from our data that Lassa virus Z protein plays an essential role in Lassa virus maturation.

An efficient method to make human monoclonal antibodies from memory B cells: potent neutralization of SARS coronavirus

Passive serotherapy can confer immediate protection against microbial infection, but methods to rapidly generate human neutralizing monoclonal antibodies are not yet available. We have developed an improved method for Epstein-Barr virus transformation of human B cells. We used this method to analyze the memory repertoire of a patient who recovered from severe acute respiratory syndrome coronavirus (SARS-CoV) infection and to isolate monoclonal antibodies specific for different viral proteins, including 35 antibodies with in vitro neutralizing activity ranging from 10⁻⁸M to 10⁻¹¹M. One such antibody confers protection in vivo in a mouse model of SARS-CoV infection. These results show that it is possible to interrogate the memory repertoire of immune donors to rapidly and efficiently isolate neutralizing antibodies that have been selected in the course of natural infection.

YY1 binding to a subset of p53 DNA-target sites regulates p53-dependent transcription

The tumor suppressor protein p53 regulates gene transcription through binding to specific DNA-target sites. We here demonstrate that a subset of these sites is targeted by another DNA-binding factor. Binding specificity, reactivity with specific antibodies, and experiments with purified protein identified the factor as the multifunctional transcription regulator YY1. The YY1 core binding sequence ACAT is present in the center of p53-half-binding sites in the p21 and GADD45 genes regulating growth arrest and DNA repair, respectively, but is absent in those of the Bax gene critical for apoptosis. In transfection experiments YY1 inhibits p53-activated transcription from the p53-binding site that contains the ACAT sequence. YY1 and p53 are colocalized around the nucleoli and in discrete nuclear domains in PC12 cells undergoing apoptosis. YY1 might attenuate p53-dependent transcription from a subset of p53-target genes and this may be relevant for directing cells either to growth arrest or apoptosis upon p53 activation.